De novo and acquired resistance to endocrine therapy are major clinical problems in the treatment of breast cancer. Two new findings from our laboratory strongly suggest that cross-talk between the estrogen receptor (ER) and the growth factor and stress signaling pathways is an important component of resistance. First, we have found in our clinical tumor bank samples that overexpression of the ER coactivator AIB1 (SRC-3) is required for the tamoxifen resistance associated with HER-2 amplification - and AIB 1 is activated by growth factor signaling via ERK 1,2 MAP kinase. Second, we have discovered in our preclinical xenograft model system that acquired resistance to tamoxifen or to estrogen deprivation is associated with activation of two stress-signaling molecules, jun N-terminal kinase (JNK) and p38 MAP kinase, which can directly or indirectly modulate ER activity. Indeed, we find that drugs targeting these pathways, already being developed for other clinical uses, not only block signaling through these kinase cascades, but also abrogate the ER phosphorylation induced by them. Our m,erall hypothesis is thus that both growth factor and stress MAPK pathways contribute to the failure of endocrine therapy by phosphorylating and activating ER and or key ER coregulatory proteins like AIB1. These pathways may thus provide predictive markers of clinical endocrine resistance. Furthermore, we suggest that the onset of resistance might be delayed or reversed by inhibitors of these pathways, resulting in more effective combined endocrine therapy. We therefore propose: 1) To determine the role of EGFR/HER-2 signaling and the ER coactivator AIBI in de novo and acquired resistance to endocrine therapy in tumors overexpressing HER-2; and 2) To define the role of stress signaling pathways in acquired endocrine resistance in tumors expressing normal levels of EGFPUHER-2. We will determine whether these mechanisms of resistance are specific for certain types of endocrine therapy with different mechanisms of action (tamoxifen, estrogen depletion, pure antiestrogens), and whether combining these ER-targeted therapies with inhibitors of these MAPK pathways or of AIB1 itself offers a promising new treatment strategy, allowing rational design of future clinical trials of this approach. Finally, we will confirm the importance of these pathways in clinical samples, and will also investigate whether these signaling molecules might serve as clinically useful predictive biomarkers for endocrine response or resistance in patients.